MM-21012: Revamp websocket implementation (#16620)

* MM-21012: Revamp websocket implementation

We replace the old gorilla/websocket implementation with the
gobwas/ws library. The gorilla library was in maintenance mode
and had a high level API due to which we cannot use that for
situations where a large number of concurrent connections needs
to be supported.

The ws library is a very low-level library that allows us
to work with raw net.Conns. We make several improvements:

- We completely remove the reader goroutines, and instead
replace them with a manual epoll implementation which sends off
messages to be read when it receives any data on the connection.
This lets us scale to a much larger number of connections.
- The reader buffer is eliminated, because we directly read
from the connection now.

https://mattermost.atlassian.net/browse/MM-21012

```release-notes
Improved the websocket implementation by using epoll manually
to read from a websocket. As a result, the number of goroutines
is expected to go down by half.
```

* fix tests

* fix shadowing errors

* final changes

* windows support!

* Remove pointer to waitgroup

* Fix edge case

* Trigger CI

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/github.com/gobwas/ws/errors.go

@@ -0,0 +1,54 @@
+package ws
+
+// RejectOption represents an option used to control the way connection is
+// rejected.
+type RejectOption func(*rejectConnectionError)
+
+// RejectionReason returns an option that makes connection to be rejected with
+// given reason.
+func RejectionReason(reason string) RejectOption {
+	return func(err *rejectConnectionError) {
+		err.reason = reason
+	}
+}
+
+// RejectionStatus returns an option that makes connection to be rejected with
+// given HTTP status code.
+func RejectionStatus(code int) RejectOption {
+	return func(err *rejectConnectionError) {
+		err.code = code
+	}
+}
+
+// RejectionHeader returns an option that makes connection to be rejected with
+// given HTTP headers.
+func RejectionHeader(h HandshakeHeader) RejectOption {
+	return func(err *rejectConnectionError) {
+		err.header = h
+	}
+}
+
+// RejectConnectionError constructs an error that could be used to control the way
+// handshake is rejected by Upgrader.
+func RejectConnectionError(options ...RejectOption) error {
+	err := new(rejectConnectionError)
+	for _, opt := range options {
+		opt(err)
+	}
+	return err
+}
+
+// rejectConnectionError represents a rejection of upgrade error.
+//
+// It can be returned by Upgrader's On* hooks to control the way WebSocket
+// handshake is rejected.
+type rejectConnectionError struct {
+	reason string
+	code   int
+	header HandshakeHeader
+}
+
+// Error implements error interface.
+func (r *rejectConnectionError) Error() string {
+	return r.reason
+}